Everyone is different regarding the correlation between sex and relationships. It is really a question, that only you can answer regarding how closely they are linked. If, in your heart, you are not ready for a relationship, and sex=one, then you are not ready for sex. That is the place I've been in for years since my divorce. I've never been wired to have casual sex, I always get emotionally attached. Since I don't want to be in a relationship, I've passed up numerous offers of sex even with friends, previous lovers, and new acquaintances. Not because I am irrationally clinging to my ex-wife, but because I just don't want to travel that road again. It is just the way I am, and that is fine. Everyone is different.

If you feel that having sex would be cheating, then no matter how illogical, it is. If having sex will result in feeling guilt about it afterwards, then no sex is good enough to be worth it. Only you can define what is "right" for you in this area. You need to decide what is the correct length of time to mourn. Others can advise you, but ultimately the choice is yours. You simply have to actively decide so that you can be at peace with that decision. There is no universal right or wrong answer, only yours.